The invention is related to a method for handling parallel and independent switching planes in a packet switch and a packet switch by means of which the method can be performed.
In order that a switch in a network for the transfer of data packets, for instance an ATM-network, will be able to satisfy the requirement of a high reliability, some form of redundancy is required, that is that some parts of the hardware of the switch are made as double or multiple units, which perform exactly the same operations and operate in parallelity to each other. A common way of providing this redundancy is by means of parallel and independent switching planes.
The transfer of data packets in a packet switch may be incurred with errors, so that for instance individual data packets disappear, will be wrongly addressed or that bit errors appear in the data packet. These errors can then be detected, when the data packet is to be forwarded from the packet switch. If several switching planes are provided, the task thus is to grasp and forward, at the output port of the packet switch, only the data packets transferred without errors and also monitor that the data packets are forwarded in a correct sequential order. In packet networks and particularly ATM-networks it is, to some extent, in addition permitted that data packets are lost or discarded. For instance, the buffers necessary for the handling of the statistical multiplexing in a packet switch can be filled, whereafter data packets arriving to the buffer may be lost. Having a redundancy in the switch, for instance with parallel switching planes, this means that the same data packet in the different planes is not available at the same time at the output port of the switch, where the redundancy is terminated, that is in the connection point of the parallel switching planes.
These different errors can, as has been observed above, be detected when the data packets leave the individual switching planes to be forwarded from the packet switch. The reliability and the communication quality can here be improved and the forwarding made quicker by selecting "the best switching plane" according to some algorithm, i.e. data packets are selected first from that switching plane which in some respect can be considered to have the best communication quality.
A change of the preferred switching plane must in this case be performed in a controlled way such that, due to the possibly varying communication velocity on the different planes, data packets are not lost or unnecessarily doubled in such a change of preferred plane.
The European patent application EP-A1 0 381 334 discloses the handling of asynchronous, redundant digital data messages. Several parallel and equivalent communication channels are each one connected to an own buffer. The various data messages which have been received in an arbitrary order are sorted and a majority choice is performed to determine the channels which have correct data messages.
In the European patent application EP-A1 0 453 607 a packet switch is disclosed having redundant switching planes. For each output port from the switch there is an evaluation unit, which receives data packets from the different switching planes and in the common way selects data packets having no errors but also selects preferably or in the first place data packets from that switching plane on which the data packets are communicated in the most slow way. For each connection there is at the output side a counter provided for each one of the switching planes where the counter for a plane, on which a packet has been correctly transferred, is increased by a fixed amount and the counters for the other planes are decreased by another smaller amount. The preferred plane from which packets will be chosen in the first place is the plane having the lowest value in its associated counter.